XCALL is how a contract on one chain calls a method on a contract deployed on another chain and gets the outcome back. Your contract emits it from inside its own code (you never broadcast one yourself); the validator federation relays the call, and the answer arrives at a callback method you name.
What you can do with it
- Read or update state that lives on a different chain, without a bridge or a wrapped token.
- Split an application across chains: price logic on one, settlement on another.
- Handle the unhappy path deliberately, since the callback always fires with a status, including when the deadline passes.